Motorcycles are known which operate by driving a rear wheel of the motorcycle via a shaft drive mechanism. Japanese Patent Laid-Open No. 2002-87364 discloses a motorcycle including a rear swing arm supporting a rear wheel. The rear swing arm is pivotally supported by left/right rear swing-arm brackets via the brackets and a pivot shaft, and is supported by a rear-wheel suspension. The rear wheel is driven by a shaft drive mechanism.
With regard to the bracket, cross members are provided so as to extend between upper portions of left/right frame members, and between lower portions thereof. An opening which opens in the fore-and-aft direction, and a concave portion a side of which is opened are provided by the frame members and the upper/lower cross members. A front end portion of the rear swing arm which is disposed in the opening is swingably coupled to the frame members via the pivot shaft. A universal joint portion of the shaft drive mechanism is disposed in the concave portion. In addition, a shock absorber is provided as part of the rear-wheel suspension and is attached between an upper connection portion (the upper cross member) of the bracket and a link attached to a lower connection portion (the lower cross member) of the bracket.
In the prior art, when the rear-wheel suspension system is assembled, the rear swing arm is bolt-fastened via the pivot shaft, and is disposed in the opening. Thereafter, the universal joint portion of a drive shaft is disposed in the concave portion opening to a side, and is coupled to an output shaft on the engine side. Finally, footrest brackets are disposed so as to laterally cover the rear swing-arm bracket by using fastenings, whereby the rear-wheel suspension system is assembled.
However, in the rear swing-arm bracket of the above motorcycle, the shaft drive mechanism is sub-assembled integrally with the rear swing arm. In the assembling process, when the rear swing arm is temporarily mounted to the rear swing-arm bracket, since a connection portion is provided between the upper cross member and the lower cross member is disposed at a position extremely biased from the center line extending along the longitudinal direction of the vehicle toward the shaft drive mechanism side, the portion of the pivot shaft near an end thereof is disposed at a position close to the shaft drive, the pivot shaft passing through a through hole in the connection portion.
In this way, a space between an end of the pivot shaft and the shaft drive mechanism is small. Thus, the space used for inserting a fastening tool, such as a torque wrench or the like, for tightening a fastening, such as a nut or the like, on the pivot shaft is scarce. For this reason, it has been difficult to insert a fastening tool into the space between the shaft drive mechanism and the connection portion to perform the work of tightening the pivot shaft via a fastening, such as a nut.
It is conceivable that, in order to avoid such a situation, an internal thread into which an external thread formed at an end of the pivot shaft is screwed is previously formed in the connection portion by a screw cutting process as disclosed in Japanese Patent Laid-Open No. 2002-87364. In this case, however, there is a problem wherein the number of steps required to manufacture the rear swing-arm bracket increases.